Good Morning TechNet!
     
     Does anyone have an answer to David Whalley's question?
     I was told (by a wise and revered source - e.g. the guy who had my job 
     before me) that there were two reasons:
     
     a) The 63/37 or 60/40 alloys were simpler to produce thus making it 
     the cost competitive choice.
     
     b) Using the 61.9/38.1 alloy didn't make sense from a practical point 
     - there is enough "contamination" of a production solder pot from 
     other metals (copper, gold,etc.) that controlling the composition 
     within a couple of percent was sufficient.
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
     


______________________________ Reply Separator _________________________________
Subject: Re: Solder Paste Plastic or Eutectic
Author:  [log in to unmask] at ccmgw1
Date:    11/25/96 7:43 AM


It would be interesting to know the history of this discrepancy between the 
real and "generally accepted" figures. Perhaps it was the result of earlier, 
less accurate measurements. In practice it may have been beneficial as 
intermetallics and oxidation deplete this "excess" tin.
     
David Whalley
     
     
>Dave Hillman is absolutely right-- the appropriate phase diagrams show that 
>the real eutectic Sn/Pb solder is 61.9%Sn/38.1%Pb (mass percent) (MP=183C), 
>just as  62.5%Sn/36.1%Pb/1.4%Ag (MP=179C) is one of the true eutectic 
>mixtures of this ternary alloy system. However, 63Sn/37Pb (Solidus=183, 
>Liquidus~185 to 186C) and 62Sn/36Pb/2Ag (S=177C, L=189C) have become the 
>'nominal' industry eutectics. In both cases the difference between solidus 
>and liquidus may be small enough, that for practical purposes the existence 
>of a pasty temperature range may be negligible. However, 62Sn/36Pb/2Ag a 177C 
>solidus--189C liquidus difference is not exactly small and could make the 
>difference between adequately wetted and inadequately wetted solder joints.
>
>Werner Engelmaier
     
*************************************************************************** 
* TechNet mail list is provided as a service by IPC using SmartList v3.05 * 
*************************************************************************** 
* To unsubscribe from this list at any time, send a message to:           * 
* [log in to unmask] with <subject: unsubscribe> and no text.        * 
***************************************************************************
     

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************